Compressor modules are already known that consist of a compressor element; a motor for driving the compressor element; if applicable a gearbox between the compressor element and the motor; a gas cooler for cooling the compressed gas and a separate closed oil circuit with an oil sump, an oil cooler and an oil pump to drive the oil from the oil sump through the compressor module, via the oil cooler and an oil filter, for lubricating the bearings and if applicable the gears.
Generally the various components are assembled in the housing of a compressor, whereby the compressed gas is cooled by coolers through which a liquid coolant such as water is driven, or the compressed gas is cooled in radiators by means of fans that make the surrounding air flow through the housing for the cooling of these radiators.
The coolers and radiators are connected by means of flexible or fixed external pipes and connections to guide the coolant along or through the compressor module. These pipes and connections are not only expensive, but are also subject to defects and leaks that can lead to higher maintenance costs of such compressor modules.
A disadvantage of air cooling is that the ventilation air that removes the heat of compression is difficult to apply usefully. It is also difficult to cool to a low temperature with air cooling because to realise a small temperature difference, also called delta T, between the ventilation air temperature and the medium to be cooled, very large coolers are required that are very expensive. For example, in practice with direct or indirect air cooling, for example by means of an oil circuit with radiator, a compressor element generally remains relatively hot at its operating speed, which is to the detriment of the compression efficiency.
An additional disadvantage is that air cooling decreases over time as radiators accumulate dust and other dirt. For this reason radiators are generally overdimensioned to always be assured of sufficient cooling capacity, and radiators must be regularly cleaned, which means extra cost.
In a large number of compressors water is used as a coolant, primarily in large compressors. Water enables the heat of compression to be recovered from a compressor and this recovered heat to be used in a useful way, for example for heating buildings or generating electric power or similar.
For such heat recovery to be economical, it is necessary for a maximum of heat to be extracted from the compressor, not only from the compressed gas and from the compressor element itself, but also from other places such as the motor, if applicable the drive of the motor, the bearings, and any gearbox, and this either directly for example by water cooling of the motor, or indirectly for example by extracting the heat from the oil that is used for lubricating the bearings and gears.
The coolers of the motor and the compressor element are generally connected to external coolers by external pipes.
The oil circuit is also always provided with a separate external cooler connected via external pipes.
A disadvantage is that a large number of external pipes and connections are needed to guide the water and the oil around and that in this case too these pipes and connections drive up the cost price of the compressor, and also bring about an extra risk of defects and leaks.